1. Field of the Invention
The present invention is related to an exhaust-gas purification system of an internal combustion engine, specifically a regeneration method of a particulate filter.
2. Description of Related Art
Recently, emission control has been required for internal combustion engines mounted in vehicles. Especially for a diesel engine, particles, such as soot (carbon black) and SOF (soluble organic fraction of particulate matter), contained in exhausted gas need to be removed in addition to CO, HC and NOx. Accordingly, a particulate filter is provided in an exhaust passage to collect exhausted particles in exhaust gas.
Exhaust gas flowing into the particulate filter passes through a porous partition wall so that the exhausted particles are collected on the surface of the partition wall and the small holes. If the amount of the collected particles excessively increases, flow resistance increases in the particulate filter and back pressure in the engine increases. As a result, engine power decreases. Therefore, the particles collected by the particulate filter need to be regularly removed in a regeneration process.
An oxidation catalyst, such as platinum, is normally provided in particulate filters so that regeneration is performed while the vehicle is operated. In this case, fuel is injected in an exhaust stroke (post injection), so that fuel is supplied to the particulate filter for removing the particles accumulated in the particulate filter. The accumulated particles are not apt to be oxidized compared with fuel. However, the accumulated particles are oxidized using combustion heat of the injected fuel in the post injection, and are removed.
If regeneration of the particulate filter is frequently performed, fuel efficiency decreases. Otherwise, if the interval of the regeneration becomes long, the amount of the accumulated particles excessively increases, and the excessive amount of the accumulated particles may be rapidly burned in the regeneration process. In this case, the particulate filter becomes excessively high in temperature, and the particulate filter may be broken. Therefore, preferably, the amount of the accumulated particles are evaluated, and the regeneration timing is determined based on the amount of the accumulated particles. According to an exhaust gas purification system disclosed in JP-A-7-332065, the flow resistance due to the increase of the amount (particle accumulation amount) of the accumulated particles in the particulate filter is detected and used for determination of the regeneration timing of the particulate filter. As the particle accumulation amount in the particulate filter increases, the flow resistance (i.e., pressure drop) of the particulate filter increases. If the pressure drop of the particulate filter exceeds a predetermined value, the regeneration is started.
However, it is difficult to precisely measure the particle accumulation amount in this exhaust gas purification system. This is because, the actual particle accumulation amount does not necessarily coincide every time, even if the engine operating condition, such as the pressure drop, is same.
Continuing, the particles accumulated in the particulate filter are partially burned due to the high-temperature exhaust gas depending on the operating condition of the engine, even before the regeneration of the particulate filter. The relationship between the particle accumulation amount and the pressure drop is different between when the particles are accumulated in the particle filter and when the accumulated particles are burned and decrease. Therefore, a measurement error can be caused in the measurement of the particle accumulation amount due to the difference between an increasing characteristic of the particle accumulation amount and a decreasing characteristic of the particle accumulation amount. The measurement error may affect the regeneration timing determination. Furthermore, if the regeneration is not completed and terminated in the previous regeneration, the particles accumulated in the particulate filter are partially burned, and the measurement error may occur.